Lennox IS quite careful in his speech. In Act 3, scene 5, Macbeth tells his wife that he has spies in every lord's house:
There's not a one of them but in his house
I keep a servant fee'd.
Consequently, it pays to be cautious in criticizing the king, especially when the consequences can be severe. Macbeth has turned against Macduff not only because of the apparition's prophesy, but also because his spies have discovered Macduff's disloyalty. In this speech, Lennox is ironically critical of Macbeth.
His words taken at face value can go for praise. He says in reference to Macbeth's killing Duncan's guards:
Did he [Macbeth] not straight,
In pious rage, the two delinquents tear,
That were the slaves of drink and thralls of sleep?
Was that not nobly done? Ay, and wisely too;
For 'twould have angered any heart alive
To hear the men deny't.
A spy reporting Lennox's words might have a difficult time proving that Lennox is being critical at all. Yet, Lennox's irony is far from subtle. Within his speech, Lennox implies that it is ludicrous to believe that Duncan's sons killed their father. He suggests in a darkly humorous way that Macbeth will probably try to pin the murder of Banquo on Fleance, because he, like Donalbain and Malcolm, fled. Lennox predicts that Macbeth would kill Donalbain, Malcolm, and Fleance if they were in his custody. He implies that killing Duncan's guards was a wise act for Macbeth because they would have denied taking any part in the killing of Duncan--why? because they were innocent!
At the end of this scene, though, Lennox drops his irony and sarcasm. He more directly refers to Macbeth as a "hand accursed" that is ruling "our suffering country." He wants Macduff to exercise caution and prays that Macduff will be able to get help from England to overthrow Macbeth.